This invention relates to fine ceramic powders and particularly the firing of such powders to convert them to a desired high temperature stable phase.
While a ceramic can be fired in an oven or a static kiln, a commercial operation that is continuous is usually preferred. One of the more versatile kilns used to fire ceramic materials is a rotary kiln comprising a long externally heated tube, usually made from a ceramic material such as silicon carbide, that rotates while in use. By changing the angle of inclination of the rotary and the speed of rotation, it is possible to vary the residence time of the material being fired in the kiln. It is also possible to vary the heating in such a way that a limited amount of control can be exercised over the temperature profile along the length of the kiln.
Such rotary kilns are widely used to fire ceramic materials that are produced in the form of particles, such as alpha alumina abrasive grits made by a sol-gel process. In such a process a dried gel of a transitional phase of alumina is fired to convert the transitional phase to the alpha phase, which has the desired abrasive characteristics.
In such a process the material to be fired in the kiln is frequently screened so as to exclude very fine powders. This is because it is found that very fine material tends to accumulate on the inside wall of the tube and not only interfere with free movement of material being fired along the tube but also to create local areas where the temperature is different, leading to thermal stress and ultimately structural failure of the tube.
The difficulties encountered increase with decreasing particle size and in general it is not considered advisable to fire powders of a transition alumina with average particle sizes less than about 65 microns, in a rotary kiln.
This means that such powders, which have many highly valuable applications from polishing slurries to magnetic tapes must be fired in batch furnaces which can require firing times of a day or more if large volumes of relatively non-heat conductive material are involved. Such furnaces need to be carefully regulated since thermal uniformity is necessary to ensure that crystal growth does not occur at local hot-spots. Such thermal variation can also lead to the formation of undesirable amounts of agglomerates. Thus production facilities for such ceramic materials often need to accommodate two different kinds of firing equipment to fire the different particle sizes.
One alternative would be to produce fine powders by firing only large particles and then comminuting the large particles, for example in ball mills, to the desired size. This is often the current procedure of choice but it suffers from the drawback that the very comminution process introduces impurities as a result of erosion of the media performing the comminution. In addition the process can take several days and is very energy intensive.
There is therefore a need for a process for firing a fine powder of a precursor of a ceramic material, such as a transition alumina, in a rotary kiln that does not seriously shorten the useful life of the tube in which the material is fired. The above need is not confined to alumina powders but extends also to other ceramic materials in fine powder form in which a key stage in the production process involves firing a precursor material at an elevated temperature.